Supreme Court Justice Stephen Hall on Tuesday jailed Francis John Wark for 18 years - six years longer than the previous record manslaughter sentence in WA.

Wark, 65, will be eligible for parole after serving 16 years but will only be released if he complies with the state's no body, no parole law.

Hayley was 17 when she was last seen alive in 1999, walking along a road near rural Badgingarra, about 200 kilometres north of Perth.

Her body has never been found.

Wark was acquitted by a jury of murdering Hayley after a six-week retrial but found guilty of the lesser charge of manslaughter.

In his sentencing remarks, Justice Hall found Wark had lured Hayley into his ute with an intention to sexually assault her and had attacked her when she tried to resist.

He then disposed of her body with "callous disregard" in a way that ensured he would not be linked to her death.

Wark was only charged in 2015 after a cold case review linked an earring and a strand of hair found in the ute to Hayley.

There were gasps in a public gallery filled with Hayley's loved ones when Justice Hall delivered the unprecedented sentence.

"Justice has been done," Hayley's mother Margaret Dodd told reporters outside court.

Ms Dodd had earlier appealed directly to Wark to reveal where Hayley's body was left, in a victim impact statement which highlighted 22 years of "sheer hell and despair" for the family.

"I wanted him to feel it," she said.

"But not a single reaction from him - a heart of stone."

Wark has been behind bars since 2007, when he was jailed for raping a woman he picked up in a remote part of Queensland in 2007.

Justice Hall said Wark's offending fell into the worst category for manslaughter given the apparent sexual motivation and Hayley's youth and vulnerability.

"She was a very young woman - no more than a girl, in fact," the judge said.

"At 17, she had her whole life ahead of her."

In a written statement read on her behalf, Rae-Anne Dodd said her older sister Hayley had disappeared a week before her 13th birthday.

She had to change the colour of her hair because people sometimes mistook her for "Hayley the missing girl".

The family held an 18th birthday party for Hayley in her absence, including a wishing well that highlighted three wishes - for Hayley to return home safe, for such a tragedy never to happen again and for "the evil monsters in our midst" to be caught.

Premier Mark McGowan last month joined the family in urging Wark to reveal where Hayley's body was left.

Wark, who denied having any involvement in Hayley's disappearance, had faced a maximum term of 20 years' imprisonment under the law that applied in 1999.

Australian Associated Press